Abstract

Magnetite (Fe3O4) is one of the transition metal (TM) oxide compounds which has promising applications in microelectronic and microwave devices as well as sensitive sensors. These possible applications are derived from the strongly correlated 3d electrons occupying the incomplete 3d orbitals in the transition metal cations. Fe3O4 has an inverse spinel structure where the Fe cations have two valence states; Fe2+ and Fe3+ ions. The Fe2+ ions reside in the octahedral (B) sites only, while the Fe3+ ions are split between the tetrahedral and the octahedral sites. This thesis will present a study of substitution, thickness and strain effects in Fe3−xCoxO4 compounds. The Co-substitution was carefully tuned to obtain films with the following stoichiometry, Fe3−xCoxO4 with x = 0, 0.1, 0.5 and 1. The discussion about the structural, electronic and magnetic properties will be presented based on comprehensive measurements. Afterwards, three different film thicknesses (5ML, 20ML, and 20nm) of Fe3O4 and Fe2CoO4 will be scrutinized to give an impression about the initial stages of the growth process. In the last part, the effect of different strain states induced by various substrates will be also discussed. All samples in this thesis were prepared by using the Molecular Beam Epitaxy (MBE) technique. The Co element as the substitution agent induces magnetic anisotropy in Fe3−xCoxO4 compounds. It modifies structural, electronic and magnetic properties of Fe3−xCoxO4 films. The amount of substitution tunes the conductivity of Fe3−xCoxO4 films. The resistivity and density of states near the Fermi energy of Fe3−xCoxO4 vary systematically upon substitution. Various techniques were employed to investigate the properties of Fe3−xCoxO4 films. Reflection High-Energy Electron Diffraction (RHEED), Low-Energy electron Diffraction (LEED) and X-Ray Diffraction (XRD) measurements were used to confirm the structure and the film quality. However, substitution Co atoms into spinel structure may reshuffle the site occupations in the structure. The oxidized Fe may have many possible valencies, i.e., Fe2+ or Fe3+, while Co ion may endure Co2+ or Co3+ valencies. These ions will reside in two possible sites, i.e., octahedral or tetrahedral. In order to investigate these issues, X-ray Photoelectron Spectroscopy (XPS) was used to check the chemical composition and the electronic structure of Fe3−xCoxO4 films. In addition, X-ray Absorption Spectroscopy (XAS) was employed to this system due to its site and symmetry selective character. Both results confirm that the Fe2+ ions in octahedral site is replaced by the Co2+ ions which are also sitting in octahedral site. Finally, SQUID magnetometery was utilized to determine the bulk magnetic properties and specify the magnetic easy axis of Fe3−xCoxO4 films. It verifies that the magnetic easy axis turns from in-plane direction for Fe3O4 films to out-of-plane direction for doped films. The thickness dependent study of Fe3O4 and Fe2CoO4 films suggests that both ferrites share a generic growth mechanism. The Fe ions in the octahedral sites are mainly populated whereas those in the tetrahedral sites are nearly empty at the beginning of growth. Sustaining the stage of film growth, the population of the Fe ions in the tetrahedral sites gradually increases while those in the octahedral sites gradually declines until they reach a balanced ratio. The Fe2CoO4 films have been grown epitaxially on three different substrates, MgO, MgAl2O4 and SrTiO3 as a model system for Fe2CoO4 under epitaxial strain. Theoretically, based on the lattice mismatch between the film and the substrate, the MgO substrate induces tensile strain while the other two substrates (SrTiO3 and MgAl2O4) induce compressive strain. The structural studies from RHEED, LEED and XRD measurements showed that Fe2CoO4 films grow coherently on MgO and MgAl2O4 substrates but on SrTiO3 substrate the growth was relaxed. Fe2CoO4 film could not follow the strain induced by SrTiO3 substrate due to a big lattice mismatch between them. As a consequence, Fe2CoO4 films grown on SrTiO3 substrate experienced tensile-like strain even though the lattice mismatch would indicate a preference for compressive strain. SQUID result showed that strained Fe2CoO4 films have an out-of-plane magnetic easy axis while compressed Fe2CoO4 films have an in-plane magnetic easy axis. This finding is also supported by the XLD results. The XLD results also explain the effect of strain on the orbital occupation in Co2+ cations. The Co2+ cations in strained Fe2CoO4 film have out of-plane orbital moment and spin moment. The other way around was observed for films grown under compressive strain. These findings give a new perspective about magnetic anisotropy behavior induced by substrate strain in Fe2CoO4 films.